The present invention relates generally to gas analyzer systems and methods of manufacturing same. More particularly, the present invention relates to a main stream gas analyzer system capable of determining carbon dioxide concentrations using low cost components.
In many applications, it is desirable to identify and monitor the concentrations of components in gas streams. For example, in the field of anesthesiology, it is often desirable to monitor the concentrations of various gaseous anesthetic or therapeutic agents, such as nitrous oxide, halothane, enflurane, desflurane, sevoflurane, and isoflurane, being dispensed to be inhaled by a patient and/or being exhaled by the patient. Moreover, it is common to continuously monitor and display the carbon dioxide (CO2) concentration relating to a patient because CO2 concentration is a good indicator of the patient""s ventilation. CO2 in the bloodstream equilibrates rapidly with CO2 in the lungs, and hence, partial pressure of CO2 in the lungs is a fair approximation of CO2 level in the blood, especially the CO2 content at the end of each breath, i.e., the end-tidal CO2. Accordingly, abnormally end-tidal CO2 level indicates poor blood flow to the tissues, inadequate CO2 transport through the lungs, or excessive ventilation. Conversely, abnormally high end-tidal CO2 level indicates that an insufficient amount of CO2 is being transported away from the bloodstream through the lungs, i.e., inadequate ventilation.
Presently, there are two types of non-dispersive infrared (IR) gas analyzers for determining gas concentrations: (1) side stream gas analyzers and (2) main stream gas analyzers. Side stream gas analyzer systems divert or draw off a portion of the patient""s inspired and expired respiratory gases from a patient""s airway of a patient circuit. This portion, or gas sample, is then transported to a distal site for analysis by a side stream gas analyzer. The analyzed gas sample is either returned to the patient.circuit or disposed of altogether.
In contrast, main stream gas analyzer systems are configured to use a portion of the patient""s airway as the sampling cell. Hence, there is no diversion of any of the gases going to or coming from the patient. A main stream gas analyzer system includes a short length of specially configured tube, i.e., an airway adapter, at the patient""s airway to act as the sampling cell. The main stream gas analyzer couples to this airway adapter such that optical and electronic components comprising the analyzer can monitor the patient""s airway. As the patient""s respiratory gases travel through the airway adapter, the desired gas, such as CO2, is monitored.
Side stream devices have a number of attractive features such as the ability to simultaneously analyze a plurality of gases comprising the gas sample; lack of device weight and size constraints; the ability to correct for gas pressure changes and the presence of interfering gases; and the ability to self-calibrate. On the other hand, having to divert a portion of the patient""s respiratory gases to use as the gas sample and also having to transport the gas sample to a distal site for the actual analysis (typically ten or more feet via a transport tube) causes distortion in the gas sample and within the patient circuit.
For example, water vapor in the gas sample will condense in the transport tube because of the change in surrounding temperature from body temperature (approximately 37xc2x0 C.) to room temperature (approximately 23xc2x0 C.). This necessitates extensive water handling systems and causes further distortion of the gas sample. Another problem is the mixing of successive gas samples as they travel toward the side stream analyzer. When an inspired gas sample mixes with an expired gas sample, information relating to actual conditions in the patient""s airway such as the inspiration-expiration transition may be lost or distorted. Still another problem is the handling of gas samples after they have been analyzed. If xe2x80x9cspentxe2x80x9d gas samples are returned to the patient circuit, they will pass through a common device, which may be contaminated by a previous patient. Thus, care must be taken to insure that returning gas samples are filtered to remove any such contaminates. Alternatively, if xe2x80x9cspentxe2x80x9d gas samples are disposed of altogether, some of the gases, such as anesthetic agents, that are intentionally dispensed to the patient will be lost. Not only are some of these gases quite expensive, but the removed gases must be monitored and be compensated for within the patient circuit so that the patient would actually be receiving the-proper amount of anesthetic agents.
Main stream devices similarly have its own advantages and disadvantages. Advantages include: (1) no distortion of gas samples because there is no diversion from nor interference with the patient circuit; (2) continuous monitoring; (3) fast response; and (4) negligible time delay from sampling to measurement display. On the other hand, because no isolated amount of gas is ever used for analysis, it is difficult to provide accurate absolute measurements. Typically, only one gas can be monitored by each main stream device. Thus, unlike side stream devices, concentrations of other gases and interference from these gases, if any, cannot be corrected. Another shortcoming is the inability to correct for pressure change. Total pressure in the patient""s airway cannot be measured and thus, pressure changes caused by compressor cycles cannot be taken into account. Still another shortcoming is that main stream devices must be small and lightweight such that it can attach onto the airway adapter without causing discomfort to or traction on the patient""s airway. It becomes very complicated and expensive to maintain the necessary dimension constraints when compensation and calibration components must also be included in order to provide a self-sustaining main stream device.
Using both types of gas analyzers would be preferable, but it is rarely done due to the prohibitive cost of each type of gas analyzer. Instead, practitioners or operators will purchase one or the other depending on the feature that is most desirable. Thus, there is a need for a main stream gas analyzer system that is small, lightweight, and inexpensive. There is a further need for a main stream gas analyzer system that is inexpensive enough to be readily used in conjunction with a side stream device. Even still further, there is a need for a main stream gas analyzer system that reduces the need to continually draw off gas samples from the patient circuit for calibration purposes, as is commonly done when the side stream device is used alone.
One embodiment of the invention relates to a low cost main stream gas analyzer system. The system includes an airway adapter including an entering plastic window and an exiting plastic window to permit non-invasive monitoring of a gas stream. The system further includes a gas analyzer coupled to the airway adapter and including a reflector. The reflector comprises a plastic component with a reflective coating.
Another embodiment of the invention relates to a low cost main stream gas analyzer system. The system includes means for interfacing with a gas stream. The means for interfacing includes an entering plastic window and an exiting plastic window to permit non-invasive monitoring of the gas stream. The means for analyzing is coupled, to the means for interfering. The means for analyzing includes a means for reflecting. The means for reflecting comprises a plastic component with a reflective coating.
Another embodiment of the invention relates to a gas analyzing system. The system includes a gas stream of interest, and a side stream system coupled to the gas stream of interest. The system further includes a main stream system coupled to the gas stream of interest. The main stream system is configured to be an inexpensive analyzer capable of placing the side stream system on standby mode. The side stream system is configured to go into active mode as needed to divert a portion of the gas stream of interest to perform calibration of a sensed signal outputted from the main stream system.